TUNICATA. 



1195 



opaque nucleoli, similar to fat-granules. The 

 external nuclei are larger, round or more or 

 less produced, and contain clear or granular 

 substance, and usually some opaque granules. 



Fig. 773. 



Test of Pliallusia mammiflaris. 



A. Transverse section, magnified 30 times. 1, inter- 

 nal layer of epithelial cells ; 2, the second or inter- 

 mediate layer, consisting of a homogeneous mass, 

 thickly strewn with nuclei ; 3, the external layer, 

 composed of a fundamental mass and cells of cellu- 

 lose, w'th dispersed nuclei (c), and having pigment- 

 cells and acicular crystals in the upper part; 4, 

 vessels. 



B. A portion of the same, magnified 350 times, a, 

 fundamental mass ; 6, cellulose cells ; c, nuclei, some 

 round, others star-shaped; d, acicular crystals of 

 carbonate of lime ; e, the extremity of a vessel ; /, 

 pigment-cells. 



The third layer forms the principal mass of 

 the test of this species. It is the seat of the 

 numerous and large arteries, which, arising 

 from the heart, traverse it in every direction, 

 having brush-like ramifications, that penetrate 

 almost to the exterior surface, and then ap- 

 pear to pass into other vessels that accom- 

 pany them in their course. This layer is 

 formed of large cells, besides a clear homo- 

 geneous substance, which is a continuation of 

 the principal substance of the second layer ; 

 besides these there are locally distributed crys- 

 tals, nuclei, and pigment-cells. 



The large cells, which R.Wagner previously 

 thought to be cartilaginous, are of a peculiar 

 nature, and resemble no other animal cell 

 hitherto known, except perhaps those of the 

 chorda dorsalis of some animals. The most re- 

 markable character of these cells is their size, 

 which varies from O'OOS"' to 0'05 /r/ , the aver- 

 age being Q'Q2'" to O'OS'". Their form is 

 spherical, pyriform, or elliptical ; their con- 

 tents diaphanous and quite destitute of nucleus 

 or granules ; and their membrane delicate, 

 smooth, and of an equal thickness throughout. 



The smallest cells are irregularly dispersed 

 in the homogeneous fundamental mass that is 

 common to the second and third layers. The 

 larger cells are arranged closely together to- 

 wards the exterior surface of the layer, pre- 

 senting a very regular cellular tissue with very 

 little intermediate substance; but, immediately 

 beneath the external surface of the envelope, 

 the cells are rather more distant one from 

 another, and the intermediate tissue more 



visible. The crystals and pigment-cells before 

 mentioned, are present only in the outer part 

 of this third layer ; the former are acicular, 

 about O'OOlo'" in length, occupying in com- 

 pact masses the intercellular intervals ; the 

 latter are yellow, and filled with somewhat 

 large granules, and surround in particular the 

 extreme ramifications of the vessels. The 

 nuclei, lastly, are similar to the large nuclei 

 of the second or intermediate layer, and are 

 everywhere present between the large cells in 

 considerable numbers. 



When slices of the test are treated with hy- 

 drochloric acid, the crystals of the second and 

 third layers quickly disappear; treated with 

 a solution of soda, the epithelial cells, the 

 nuclei, the pigment-cells, and the vessels are 

 dissolved. The fundamental homogeneous sub- 

 stance of the second and third layers and the 

 large cells are not dissolved, nor do they 

 suffer any modification. 



In Phallusia monachus the large cells mea- 

 sure from O'Ol'" to G'02"', and are more dis- 

 tinctly separated from one another than in the 

 preceding example. The nuclei of the homo- 

 geneous substance are few, and generally fusi- 

 form or even ramified ; at the external surface 

 of the envelope they are mixed up with a 

 great number of minute yellow pigment-cells 

 and pigmentary granules, as well as with acicu- 

 lar crystals and very minute crystalline concre- 

 tions ; all of these being in the greatest num- 

 bers in the neighbourhood of the extremities 

 of the vessels. In one specimen of this spe- 

 cies MM. Lowig and Kolliker observed, that 

 in the interior of the third layer no cells could 

 be distinguished ; ultimately, however, they 

 distinctly saw well defined cavities or lacuna?, 

 which were evidently vestiges of cells that had 

 been more or less completely fused with the 

 intermediate homogeneous substance ; and the 

 traces of these lost cells were found to be 

 more and more distinct towards the band of 

 perfect cells in the surface of the test. 



In Phallusia sulcata the large round or ellip- 

 tical cells, without nuclei, have a diameter of 

 O-o 1'" 0-15'". 



In Phallusia gclatinosa a ver} r peculiar forma- 

 tion was observed by the experimenters. In 

 one specimen the soft, gelatinous substance 

 of the envelope exhibited no trace of cells 

 throughout its thickness, but its mass was 

 principally composed of a homogeneous sub- 

 stance similar to that of the other Ascidue. In 

 another individual they observed some few 

 and indefinite remains of cells. In both spe- 

 cimens they found, as in the other species, 

 vessels and nuclei, the latter for the most part 

 round and measuring Q'002'", in the homo- 

 geneous substance. In the individual desti- 

 tute of cells, there was also in the exterior 

 part a very large quantity of acicular crystals 

 and yellow granules, the latter frequently re- 

 sembling the nuclei with large coloured nu- 

 cleoli. The tissue in all the Asndue examined, 

 when chemically treated, behaved in a similar 

 manner to that of Phallusia mammillaris. 



We have observed that the test of Boltenia 

 rcnifurmis (preserved in spirit) presents a ho- 



